The design of an immunogen able to elicit broadly neutralizing antibodies is a major, but so far elusive, goal of HIV vaccine research. The existence of such antibodies is indicated by the description of rare broadly neutralizing HIV sera and a handful of broadly neutralizing human monoclonal antibodies (bnMAbs) isolated from HIV infected individuals. To understand how to better induce broadly neutralizing HIV antibodies it is important to identify the appropriate B cell precursors and to stimulate them with effective antigens. For this reason we propose to develop mouse strains carrying B cells with "germline" versions of bnMAbs and to follow their responses to test immunogens. This approach renders visible the responses of the very B cells that we wish to recruit into the HIV antibody response and should allow rapid screening of vaccine candidates for the ability to elicit neutralizing antibodies in a context in which the appropriate precursors are present in high frequency. Failure of response in this context would not be ascribable to a deficiency in appropriate B cells unless such B cells are eliminated by immune tolerance, an outcome that can be experimentally excluded. The specific aims of the proposal are (1) to generate transgenic (knockin) mice carrying B cells expressing germline versions of the bnMAbs b12, 2G12 and 2F5 and (2) To investigate antibody responses in the transgenic mice to a wide variety of immunogens using a number of immunization strategies. The project is a close collaboration between a laboratory (Nemazee) with expertise in B cells, including knockin mice, and one (Burton) with expertise in antibodies and HIV. It is expected to generate valuable reagents and animals for the proposed UO1 consortium.